Recently, the demand for reliability of semiconductor devices is increasingly growing, and in particular improvement of heat cycle resistance properties is required for a junction between a semiconductor element and a circuit board having a large difference in thermal expansion coefficient. Conventionally, semiconductor elements having substrates made of silicon (Si) or gallium arsenide (GaAs) have been commonly used, and they are operated at a temperature of 100° C. to 125° C. As a solder material joining these elements to electrodes of an electronic circuit, 95Pb-5Sn (% by mass) has been used for Si devices and 80Au-20Sn (% by mass) or the like has been used for gallium arsenide devices, considering crack resistance to repeated thermal stress due to a difference in thermal expansion between a semiconductor element and a circuit board, a high melting point for dealing with multi-stage solder joining used during assembly, pollution resistance of the devices, and the like. However, 95Pb-5Sn containing a large amount of harmful lead (Pb) is problematic from the viewpoint of reducing environmental load, and an alternative to 80Au-20Sn has been strongly desired in terms of soaring prices of precious metals and reserves thereof.
On the other hand, development of devices having substrates made of silicon carbide (SiC) or gallium nitride (GaN) as next-generation devices has been actively pursued, from the viewpoint of energy saving. These devices are designed to be operated at a temperature of not less than 175° C. from the viewpoint of reducing loss, and it is said that they will be operated at 300° C. in the future.
For the above demand, a high-temperature solder alloy having a high melting point and excellent heat resistance is required. Such a high-temperature solder alloy is disclosed, for example, in Japanese Patent Laying-Open No. 2004-298931. In the publication, a lead-free, high-temperature solder alloy composed of 10 to 40% by mass of Sb, 0.5 to 10% by mass of Cu, and the remainder including Sn, and an electronic component joined using the solder alloy are disclosed. In addition, a solder alloy containing Ni, Co, Fe, Mo, Cr, Mn, Ag, and Bi as elements for improving mechanical strength is disclosed. Further, a solder alloy containing P, Ge, and Ga as elements for suppressing oxidization is disclosed.
In addition, a technique for obtaining solder joining strength that is satisfactory even at a high temperature of 200° C. is disclosed, for example, in Japanese Patent Laying-Open No. 2007-67158. In a semiconductor device according to the publication, materials to be connected plated with Ni-based plating are connected using an Sn-based solder foil containing a phase of a Cu—Sn compound (for example, Cu6Sn5) at a temperature from room temperature to 200° C., and thereby a compound layer mainly composed of the Cu—Sn compound is formed. The compound layer serves as a barrier layer for the Ni-based plating and the Sn-based solder, and suppresses growth of the compound by a reaction at a connection interface. Further, in the examples, a solder foil in which not less than 3% by mass of Cu is contained in Sn or Sn-3Ag (% by mass) is brought into contact with the Ni-based plating, heated and melted, and thus joined.